1. Field of the Invention
This invention related to a method for the production of an absorbent resin. More particularly, it related to a method for economically advantageous production of an absorbent resin which possesses a firm particulate structure even when it is wetted with an absorbed liquid in spite of the origin thereof in recycled fine particles of absorbent resin, and excels in the handling property and various absorbent characteristics.
2. Description of the Prior Art
In recent years, the absorbent resin has found utility in various fields involving sanitary materials such as disposable diapers and sanitary napkins, agricultural and horticultural water-retaining agents, and industrial dehydrators. Generally, when the absorbent resin is to be used as these various uses, there are the optimum particle diameter of the absorbent resin therefor the used. The conventional absorbent resin generally contains an appreciable proportion of fine particles having diameters smaller than the diameter most favorable for the application.
The use of the absorbent resin of this quality is, however, found following problems.
(1) The absorbent resin, while being handled, tends to emit dust and consequently jeopardizes the work environment and suffers from waste of supply.
(2) The absorbent resin, while absorbing a liquid, tends to form fish eyes and obstructs diffusion of the liquid and, when used as in a disposable diaper, tends to entail leakage.
(3) The absorbent resin, when to be mixed with other substance, exhibits poor mixability or dispersibility and defies uniform mixture as with soil.
(4) The absorbent resin possesses poor flowability and, therefore, tends to form bridges of resin particles inside a hopper and entail the phenomenon of flushing.
As measures for solution of these problems, techniques for the aggregation of absorbent resin and techniques for recycling of fines form the absorbent resin have been reported. As the techniques for aggregation, for example, a method which effects aggregation of fine particles of absorbent resin through water or water and a water-soluble polymer or water and inorganic powder [JP-A-61-97,333 (1986) and JP-A-61-101,536 (1986)], a method which effects aggregation of an absorbent resin fine powder by dispersing the powder in a organic solvent, adding an aqueous solution of monomer to the resultant dispersion thereby allowing the dispersion to absorb the aqueous monomer solution, and then polymerizing the monomer [JP-A-62-230,813 (1987)], and a method which attains aggregation of an absorbent resin power by preparing an organic solvent containing a hydrated gel of absorbent resin obtained by the reverse-phase suspension polymerization technique, adding the absorbent resin fine power to the organic solvent, and subjecting the resultant mixture to azeotropic dehydration [JP-A-63-210,108 (1988)] have been known to the art. Then, as the techniques for recycling of fine powder, a method which effects recycling of an absorbent resin powder by swelling the fine powder with an amount of water enough to form a continuous amorphous gel and crushing the dried gel (U.S. Pat. No. 4,950,692), a method which accomplishes recycling of an absorbent resin powder by mixing the powder with an aqueous solution of cross-linking agent thereby forming a gel-like mass, subjecting the mass to a cross-linking reaction, drying the cross-linked mass, and crushing the dry mass (EP-A-401,044), and a method which attains recycling of an absorbent resin powder by mixing the dry powder and the water simultaneously with the spraying by the use of shear stress high enough for the powder to be substantially completely hydrated and discharged from the site of the mixture without inducing flocculation, then intimately blending a viscous material with the hydrated powder in a separate site of mixture with shear stress of a medium degree capable of producing a substantially homogenous product (EP-A-0 417,761), for example, have been known to the art.
The aggregates produced by the methods of JP-A-61-97,333 (1986) and JP-A-61-101,536 (1986), however, have no sufficient strength and, while being conveyed in a plant line or on a vehicle, have the possibility of sustaining partial fracture and consequently regenerating fine particles. By the method of JP-A-62-230,813 (1987), the whole absorbent resin uniformly absorbs the aqueous solution of monomer only with difficulty, the aggregation is formed in a low proportion to the whole powder used. And the aqueous solution of monomer is polymerized as excessively penetrated in the resin powder, the absorbent resin consequently suffers from a lowered absorptive capacity. Though the method of JP-A-63-210,108 (1988) indeed is capable of producing aggregate particles, the particles produced thereby at all have low agglomeration strength, regenerate fine gel while being swelled with absorbed liquid, and retard permeation and diffusion of liquid.
Since the methods disclosed in U.S. Pat. No. 4,950,692, EP-A-0 401,044, and EP-A-0 417,761 require the absorbent resin powder to be preparatorily swelled by addition thereto of water or an aqueous solution of cross-linking agent and then the swelled resin powder to be dried, they are not only complicate and difficult in point of operation but also disadvantageous in point of economy. The dry particles which are formed solely of the absorbent resin powder produced as taught in U.S. Pat. No. 4,950,692, when they are swelled with absorbed liquid, suffer substantially all the fine particles used therein to be regenerated in the form of fine gel and, therefore, are deficient in the ability to permit permeation and diffusion of liquid. The method of EP-A-0 401,044 has the disadvantage that since the cross-linking agent permeates the fine particles of the absorbent resin powder to their cores in conjunction with a large amount of water and effects a cross-linking reaction therein, the dry powder to be eventually produced suffers a very poor absorptive capacity.
An object of this invention, therefore, is to provide a novel method for the production of an absorbent resin.
Another object of this invention is to provide a method for economically advantageous production of an absorbent resin which possesses a heretofore unattainable firm microstructure even while it is being swelled with absorbed liquid in spite of the origin thereof in recycled fine particles of absorbent resin and excels in the handling property and various absorbent characteristics.